The number of patients with allergies, such as atopic dermatitis and pollinosis, is increasing every year. According to the results of the investigation conducted by the Ministry of Health, Labour and Welfare of Japan, one in three people have an allergic disease of some kind in Japan. As causes of the increase in allergic diseases in recent years, increased allergens, such as pollens and mites, environmental pollution, an increase in intake of protein due to the Westernization of dietary habits, a change in intestinal bacterial environment, stress, and a decrease in infection by bacteria and parasites (i.e., the hygiene hypothesis) are illustrated. Along with allergic diseases, inflammatory bowel diseases are rapidly increasing in Japan, such diseases are closely related to the intestinal environment, and, as causes of development of such diseases, disturbance in the intestinal environment and abnormalities in the gut immune system in which type II helper T cells (Th2) become dominant over type I helper T cells (Th1), have been suggested.
The intestinal tract comprises gut-associated lymphoid tissue (GALT) composed of, for example, the Payer's patch (PP), the lamina propria (LP), the lamina propria lymphocytes (LPL), the intraepithelial lymphocytes (IEL), the intestinal epithelial cells (IEC), and the cryptopatch (CP), and the intestinal tract functions as the greatest immune organ in the body. In particular, microfold cells (M cells) existing in the follicle-associated epithelium (FAE) that covers the lumen side of the Payer's patch are specialized in antigen uptake, and M cells play a key role in the induction of immune responses of the intestinal tract. The molecular mechanism associated with antigen uptake from the M cells has gradually been elucidated. As GP2 expressed in M cells binds to the bacterial antigen or the like, the antigen is transferred to the immunocytes (e.g., antigen-presenting cells, B cells, and T cells) existing inside the Payer's patch, and various immune responses, such as the production of IgA antibody or cytokines, take place (Non-Patent Document 1).
As described above, allergic diseases and inflammatory bowel diseases are considered to result from disorders in the gut immune system. Accordingly, it is considered effective for the gut immune system to be activated in order to normally function, so that the diseases described above would be treated and prevented. To date, a variety of food raw materials that can control the intestinal environment and improve the gut immune system have been examined, and lactic acid bacteria are among the most effective food components. It has actually been reported that the immune balance is improved with the administration of lactic acid bacteria and that lactic acid bacteria are effective for improvement of allergic diseases and inflammatory bowel diseases (Non-Patent Document 2). However, the effects of lactic acid bacteria vary depending on the type thereof, and lactic acid bacteria with stronger immunoregulatory functions, such as anti-allergic functions, have been desired for probiotic food.
In order to utilize lactic acid bacteria exerting various functions, such as immunoregulatory functions, intestinal regulation functions (e.g., acceleration of intestinal peristalsis, regulation of intestinal flora balance, remediation of diarrhea and/or constipation, and stool odor reduction), life-extending actions, improvement of nutrient digestion and absorption, anti-aging actions, pathogen-eliminating actions, cholesterol-lowering actions, stress relief, cutaneous function improvement, beauty effects, anti-inflammatory actions, cancer inhibition, and dental caries prevention, it is necessary to conduct clinical trials involving human subjects so as to verify the functions of such lactic acid bacteria. To this end, a means for primary screening of useful lactic acid bacteria in a more simple and rapid manner is necessary. Up to the present, methods for screening for lactic acid bacteria, such as methods involving the use of animal models of allergic diseases (Patent Document 1 and Non-Patent Document 3) and methods involving the use of cultured cells (Patent Documents 2 and 3) have been known. However, screening methods involving the use of disease animal models are time consuming, they necessitate laborious procedures, and it is difficult to prepare many test groups. Also, the results attained for the established cell lines that are frequently used for a screening method involving the use of cultured cells cannot always be reproduced with primary cultured cells. In addition, inherent regulatory mechanisms may occasionally be denatured during the process of cell establishment, and, disadvantageously, in vivo reactions are not always reflected (Non-Patent Document 4). Therefore, a method that can be used to extensively screen for lactic acid bacterial strains with strong immunoregulatory functions from many lactic acid bacterial strains in vitro in a simple and rapid manner and with a single operation has been awaited.